DE 199 03 439 A1 describes a procedure for operating an internal combustion engine, in whose exhaust gas area a SCR-catalyst (selective-catalytic-reduction) is arranged, which reduces the nitrous gases that are contained in the exhaust gas of the combustion engine with a reagent into nitrogen. The metering of the reagent or a precursor of the reagent preferably takes place depending on operating parameters of the combustion engine, as for example the engine speed and the injected fuel amount. Moreover the metering takes place depending on exhaust gas parameters, as for example the exhaust gas temperature or the operating temperature of the SCR-catalyst. The reducing agent ammoniac is for example provided as the reagent, which can be won form a urea/water solution. The metering of the reagent or reactants of the reagent has to be determined carefully. A metering that is too low causes that nitrous gases cannot be completely reduced anymore in the SCR-catalyst. A metering that is too high causes a reagent slip, which can cause an unnecessary high reagent consumption on the one hand and, depending on the kind of reagent, an unpleasant odor nuisance on the other hand.
The determination of the nominal flow or the metering amount can take place according to EP 1 024 254 A2 based on an operating parameter of the combustion engine like the fuel injection amount and/or the engine speed and at least on parameter of the exhaust gas if necessary, as for example the exhaust gas temperature.
DE 10 2004 031 626 A1 describes a method for operating a SCR-catalyst that is used for purifying the exhaust gas of an internal combustion engine, which provides a control or regulation of the reagent filling level in the SCR-catalyst on to a preset storage nominal value. The targeted default of the storage nominal value ensures on the one hand that a sufficient reagent amount is provided in unsteady states of the combustion engine for an almost complete elimination of the NOx raw emissions of the combustion engine and on the other hand that a reagent slip is avoided. The reagent filling level of the SCR-catalyst is determined with the aid of a catalyst model, which considers the NOx mass flow that flows into the SCR-catalyst, the NOx mass flow that leaves the SCR-catalyst, the catalyst temperature and if necessary the reagent slip. The maximally possible reagent filling level of the SCR-catalyst especially depends on the operating temperature of the SCR-catalyst, which is the highest at lower operating temperatures and drops to lower values with increasing operating temperatures. The efficiency of the SCR-catalyst depends on the catalytic activity, which is low at low operating temperatures, passes a maximum with an increasing operating temperature and drops again with a more increasing operating temperature.
The reagent or the precursor of the reagent is metered with a metering valve, which for example can be arranged at an exhaust gas channel of the combustion engine. For purposes of the mechanical reliance of the metering valve and with regard to an expedited ageing process of the reagent accompanied by a possible corrosion affinity a cooling of the metering valve can be provided. Such a cooling is described in DE 44 36 397 A1, which provides a cooling jacket that is incorporated into the cooling water circuit of a combustion engine and that surrounds the metering valve.
The invention is based on the task to realize a possibly necessary cooling of a metering valve by simple means.